How to build your own surfcraft hydrofoil.

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I found the original design didn't provide enough lift to get a surfboard + me out of the water in the conditions I was in. I started working on a larger wing but I haven't tried it yet. I did feel some lift and it didn't feel squirrelly but I haven't put any time into it.

Hi Gdaddy, I think there’s a minimum wave energy to get lift. I’d like to say you can foil on any wave but the wave needs some Go and face to run on.

I think ‘ foiling ‘ needs a definition.

There’s the foiling that is currently everywhere, wide foiled front wings on small waves, flying at speed and pumping form wave to wave. It’s excellent and looks great.

Then there’s the original foiling since the 70’s which was to ride a foiled craft in the normal surf as you would a shortboard. Beach or reef breaks but on steep faced pitching waves, bottom turns, pulling into tubes, stalls, snaps, all the things that pioneers Terry Hendricks and Gilbert Lum worked to create. Terry was the absolute king of foiling science and design.

Theres a big difference between the first foiling that relies on smooth water with a slight wave that relies of the foiled wing for lift (Bernoullian) and the second that relies on the rising force of the wave face for lift. ( Newtonian)

Both rely on both theories for lift but each is at opposite ends of the spectrum. The wide foils won’t work in jumpy pitching tubes and my flat, low aspect foils give poor lift in marginal conditions. Bu both have proof that they function. This from me 5 years ago, after Laird, but 3 years before Kai, and 30 years behind Terry.

I know there’s vid of Pro surfers riding high aspect foils in the surf but that more about skill than design, theres also vid of Slater riding a dining table which doesn’t prove we should all ride tables.

I found that I didn’t need the highly tuned foil to ride in normal surfing waves, it just doesn’t need all the high tech design. There’s so much lift from the rising wave face that, and I’ve said it many times , foiling in normal surf is all about reducing lift. And you see in the videos how many times a small foil bursts through the surface.

The ‘holy’ foils work better in steep waves providing a variable lifting surface that specifically designed for hollow waves. As I said in another thread that Stoneburner mentioned, there’s a value in what you leave out as much as what you put in. You can incorporate something that doesn’t work to get a better overall design. In a submerged situation on a barreling wave there’s no way you can put a thick high aspect wing in there and hope it will work. Pulling into a head high hollow wave, you need a much smaller foil to handle the massive lift.

In the end Terry understood my theories and videos and copied my low aspect foils and that’s enough validation for me and I hope other foil makers try my version of foil design.

Currently I’m still making the holy foils in laminated ply and carbon but incorporating FCS plugs and testing commercial surfboard fins into my foils for better grip and hold in the tube.

SGO, I think the foiling thing now is like the first few years of the Thruster back in the 80’s. Lots of new ideas floating about as well as a healthy dose of criticism. Interestingly foil guys here were probably some of the first ever.

A foil is a fin but a horizontal fin, giving true lift as we understand vertical lift, and also gives previously unknown speed and acceleration on the least significant waves. It’s an amazing science that’s now on the waves.

I think it’s got a long way to go. Pun intended.

I don’t like the look of the Twin Tip foils with the big wide slicing front foil that’s currently popular, it has so many disadvantages I’m amazed it’s still available. However, with enough interest people will look, as they did with the Thruster, for new variations.

The difference with the Thruster is that its evolution was driven by the surfers to create something that surfed better whereas foiling seems to be driven by profit. There’s so many foil companies all producing the same product to service the same market but there’s no surf-foiling competition to promote evolution.

Anyone can start building a foil and make great progress because it’s still in its infancy, even with all the vacced carbon flying around. High tech materials are all well and good and a huge media profile is lovely if you’re marketing a generic product but

A Surfboard is a single item but foiling requires a craft and struts and a foil/s.

the hardest part for me is working out how to connect these components..

Ive tried all the basic screw / bolt / glue methods of connecting parts but I enjoy using FCS plugs. Mainly because they are familiar to the industry and also because they are very versatile so you could arrange a set of plugs in any configuration. And flexibility is a bonus when you are going to redesign over and over again.

Pic 1. An older foil that has a Left and Right longitudinal Foil because on a steep wave the outside foil, much like the outside rail on a surfboard, means nothing. The front cross beam is more for stability and preventing ventilation from sucking across the foil and the crude but effective Aluminium fins gives low volume grip in the tube. Without the fins there’s nothing to resist the upward flow of the wave and you either lose speed or go over the falls. You can foil without fins, but on proper waves you need fins on foils for the same reason of hold and drive that you need fins on surfboards.

Incorporating standard surfboard fins into foil design is a godsend, where else could you work on a brand new way to surf but already have part of the answer already solved. Irrespective of the craft, fins are so highly developed that they are the perfect part for every surfcraft.

Pic 2. Polycarbonate struts. If I’m going to use FCS plugs on the craft I may as well standardise with FCS plugs on the struts and foils. polycarbonate is the bulletproof plastic they use in bank screens so it’s not going to snap or shatter. And with both ends of the struts cut to suit plugs I only have to put plugs in the foils to finish the system.

But that’s not so easy. The really thin foils I use are 5 mm and are very fast because it has little volume to displace water, it moves with little resistance but an FCS plug is design3d to be embedded in a surfboard.

Pic 3 & 4. So I slaughtered a few plugs in the name of science to see if I could make a plastic plug fit into a foil. And then realised an FCS plug is just a a slot and a hole so I tried to replicate that exact system on a ply foil.

Pics 5 & 6. it turned out a complete failure but I learned that the system of a screw holding a fin is easy to replicate and can work in many ways that’s just as effective.

Any day of the week (that's there's at least 1-2 foot surf in Waikiki) I can lounge around and watch hydrofoil SUPs and prone paddlers, foiling a couple hundred yards. Once up on th foil, some of the waves they keep going on are nearly imperceptible. That they can get far in front of a wave, or run along the crest, suggests they aren't at that point in time relying on Newtonian forces (generated by water rushing upwards in the wave face), they're flying Bernoulli from lift generated from the foil shape.

So looks to me like the design work has already been done. And what do I see? Molded foils with about 30" span 10" to 12" chord at center, a boom some 30" long with a trailing foil, a strut some 3 feet tall that has an 8" chord. That's what works.

Wanna re-invent the wheel (or foil), go ahead but I'm not about to pay $1200 or something like, to try and replicate success that's staring me in the face. But I bet a much smaller amount that I can nearly duplicate that rig and get going, at least prone, far before my engineer mind can dig up the lift data, draw it out, shape or hotwire, glass, install plugs/boxes, and get wet.

One other thing... the suggestion to wrap an EPS or similarly easily-shaped but weak foam in a layer of veneer, (and a couple layers of glass) is interesting from an appearance standpoint, but beware the shear distribution in such a sandwich construction. You see pretty quickly the foam/veneer interface weakness. The failure will be along the underside of the veneer. This is one of the major mechanisms for lightly glassed surfboards to break. Not the only way, and I haven't collected a large number of broken boards to properly gather the data, but a shear plot would show you pretty quickly how weak that construction is at the interface. The old shear demonstration with the deck of cards is the beginner's way to understand shear. It gets more complex, quickly, when materials of different levels of shear resistance are involved.

I believe that for any given foil setup there is a minimum wave speed needed to become foilborne. And since wave speed is directly related to height, it boils down to that a small wave needs a foil with a lot of lift. You can get some small advantage on a steep wave because the uprushing water affords lift, but for the kind of waves most foilers should start the experience, I'd suggest something with a two to three foot face will be minimum. Unless you can hump it like Kai, but then again and as mentioned, just because Slater can surf a tabletop doesn't mean the rest of us can. Perhaps therein lies the danger of seeing someone else do (something).

For a beginner builder the quoted area/mass ratio on the previous page would be a good starting point. I'd also like to see it validated, but any few moments spent measuring what I see on the beach will do that.

Apparently Gaylord Miller, at Scripps Institute of Oceanography, created a "flat foil" that worked well enough to lift his 200-lb+ body (and board) out of the water in the early 1960s. (Diagram and photos at bottom of post.)

Brett appears to have adapted a simple technology that worked more than 50 years ago.

A veneered EPS foil -- a flat wooden "core" with EPS added to one (or both) side(s) to allow shaping of a NACA style foil. Wood core thickness, between foam layers, would have a significant effect on foil stiffness. Adding veneer skins should add more strength to the composite sandwich. I will assume attaching a mast directly to the middle wood plate/core of the foil, rather than the skin, changes the stress dynamics.

A 2-foot plunging wave has a speed of approximately 6.3 mph. A 1-foot plunging wave has a speed of approx. 4.4 mph. (Speed for spilling waves would be approx. 15% faster.) But, in addition to wave height, critical velocity for takeoff will also be affected by the slope of the wave's face -- physics of curved ramps/inclined planes.

"Surface area of 2.5 - 3.1 sq. in./kg at ~ 3 mph (5 kph) for lift. Quarter this for each doubling of speed. However, you need the larger surface when you take off to initiate lift at low velocity. As velocity climbs, you need less surface area."

Hi Bill, yes that’s the interesting thing about the recent invention of surf foiling that’s 50 years old.the new kids know all but know little about the history of surf foiling, and I mean foiling at your average beach or reef break, just the local beach wave that 99.9% of surfers ride.

The original foil that Laird rode came from an ‘Air Chair’, a sit down tow behind a boat on flat water foil. He made it work in massive surf but it had no relevance to the surfing community, the other 99.9%.

Then the company that pays Kai, GoFoil, brought out a foil that does an excellent job but only in the marginal conditions that average surfers wouldn’t bother to ride.

What the foil industry misses is that the 99.9% would like something that they can use at their local beach, not something that needs massive or tiny waves because part of the surfing experience is the satisfaction of mastering conditions.

And that something is already invented. By Terry Hendricks, Gilbert Lum, Gaylord Miller, me and heaps of other people. It’s a flat, slightly foiled, low aspect foil. Not overly exciting but it does the job and it does it better in the surf than any modern foil. I get the feeling the modern foil guys just don’t want to believe what they see, somehow if they ignore the 50 years of evidence then they can say they invented foiling. It’s just dumb.

I didn’t fake my videos but they clearly show a guy consistenly foiling on an average wave. Same with Terry Hendricks’ 40 years of documentation and he had a patent out for a surf foil. It’s just amazing that with such an opportunity to make money with this new sport the new crew chose to start fresh and not capitalise on the clearly successful R and D that went before.

But what even more interesting is that the new foil guys started out with long boards attached to heavily foiled, high aspect foils and 3 foot masts but are now moving towards the designs that actually work in real surf.

The boards are dramatically shorter.

The masts are down from 3 foot to 1 foot.

The foils are thinner.

The aspect ratio is lower.

With lower A/ R the back foil is slowly eliminated.

And there are astute designers moving towards very low aspect foils with an inclination towards a left / right split foil system like the ones that I and David Tomasetti pioneered ove fthe past 10 years. All the early foil guys used the shortest surfcraft available, typically prone craft.

Give the industry a few years and they will have ‘invented’ new foil designs that will be very similar to what was done 50 years ago,

Regarding Laird Hamilton, I believe he was being towed into waves to achieve take-off velocity for foiling. Tow-in would be more like the "boat chair" you mention.

Perhaps, a mast system that has interchangeable foil capabilities is the solution to foiling in different wave conditions. Change out foil shapes/sizes for different conditions, a bit like changing out fins and/or board size and shape.